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The C# compiler has an MIT license and is available on GitHub, which is about as FOSS as it gets.

The compiler is one thing, the ecosystem is another.

Sure, you can compile and host your .NET Core web app on Linux these days, but desktop applications are a completely different beast. You're often lucky if one exists at all, and if it does there's a decent chance it's not designed for "modern" C#-on-Linux. It really isn't a viable option yet, maybe in a decade or two.

WPF is MIT licensed, available on github. The fact that it's windows only doesn't make it less FOSS. Microsoft doesn't have incentive to port it to other platforms but other people and companies are free to do that.

WPF is not the best example of open source, as some components are still closed source. Though it only runs on Windows, a closed source operating system, so perhaps that is not so important.

https://github.com/dotnet/wpf/issues/2554

That said, there are cross platform, open source .NET UI frameworks out there, including one that is inspired by WPF:

https://avaloniaui.net/

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